STM+ Sleep training while room sharing

cchill01

My DDs are in the same bedroom.  The baby was sleeping through the night, so I thought we were good.  But, now she's waking every hour to three hours.  The only way I can keep her sleeping is in bed with us - which I am really opposed to doing. 

How did you sleep train #2, with #1 in the room?  My three year old is a very sensitive sleeper, so I don't want to mess up her night.  Every time #2 cries, I run in there because I don't want to wake #1.